The present invention relates to an optical pickup device provided with a plurality of laser beam emitting sources.
Conventionally, as an optical pickup device for performing recording and reproduction of an optical disk, there is one that can cope with two types of optical disks. This kind of optical pickup device is required to use laser beams of different wavelengths according to the types of the optical disks. For example, when executing both recording and reproduction of a CD and a DVD, the optical pickup device is provided with a first laser device that emits a laser beam for executing write and read of data on the CD and a second laser device that emits a laser beam for executing write and read of data on the DVD.
FIG. 28 shows a schematic view of the aforementioned conventional optical pickup device. This optical pickup device has a first light source 211 that emits a first laser beam of a wavelength λ1 for data reproduction, a second light source 212 that emits a second laser beam of a wavelength λ2 for data reproduction and data recording, an optical separator 213, an optical monitor 215 for the second light source and an object lens 214.
The optical separator 213 is formed by bonding one side surface of a first triangular prism 216 located on the first light source 211 side to one side surface of a second triangular prism 217 located on the second light source 212 side, with a wavelength selecting film 218 being placed therebetween.
The wavelength selecting film 218 of the conventional optical pickup device transmits only several percent of the first laser beam of the wavelength λ1 emitted from the first light source 211, while reflecting most of the first laser beam of the wavelength λ1 emitted from the first light source 211. On the other hand, the wavelength selecting film 218 reflects only several percent of the second laser beam of the wavelength λ2 emitted from the second light source 212 while transmitting most of the second laser beam of the wavelength λ2 emitted from the second light source 212. Then, the several percent of the second laser beam reflected by the wavelength selecting film 218 is made incident on the second light source optical monitor 215 to execute control of the optical output of the second light source.
With the aforementioned construction, when data written on a loaded optical disk 210 is read by the first laser beam of the wavelength λ1 emitted from the first light source 211, the laser beam emitted from the first light source 211 is reflected by the wavelength selecting film 218 toward the object lens 214 and thereafter concentrated on one point of the optical disk 210 by the object lens 214. Then, data (pit information, for example) of the optical disk 210 is extracted by a hologram element, a light-receiving element or the like (not shown) from the first laser beam that was reflected from the one point of the optical disk 210 and passed again through the object lens 214. In order to control the optical output of the first light source 211, this optical pickup device detects the quantity of light of the laser beam emitted from the first light source 211 by means of an optical monitor (not shown) for the first light source, the monitor being provided inside the same package as that of the first light source 211.
On the other hand, when data written on a loaded optical disk 210 (even in the case of a different optical disk, the optical disk is denoted by the same reference numeral 210) is read by the second laser beam of the wavelength λ2 emitted from the second light source 212, the second laser beam is transmitted through the wavelength selecting film 218 and thereafter concentrated on one point of the optical disk 210 by the object lens 214. Then, the data written on the optical disk is extracted by the hologram element, light-receiving element or the like (not shown) from the second laser beam that was reflected by the one point of the optical disk 210 and passed again through the object lens 214.
When data is written on a loaded optical disk 210 by the second laser beam of the wavelength λ2 emitted from the second light source 212, the data write is performed by transmitting the second laser beam emitted from the second light source 212 through the wavelength selecting film 218 and thereafter concentrating the beam on one point of the optical disk 210 by means of the object lens 214. In order to control the optical output of the second light source 212, the optical pickup device detects the quantity of light of the second laser beam emitted from the second light source 212 and reflected by the wavelength selecting film 218, by means of the second light source optical monitor 215.
Optical signals detected by the first light source optical monitor and the second light source optical monitor 215 are converted into respective electrical signals, and these electrical signals are inputted to an automatic power control (APC) circuit (not shown) provided on the optical pickup device or outside the optical pickup device. Then, outputs of the automatic power control circuit, corresponding to the optical signals of the first light source optical monitor and the second light source optical monitor 215, are fed back to the first and second light sources 211 and 212 to control the optical outputs of the first and second light sources 211 and 212.
However, in the aforementioned conventional optical pickup device, several percent of the second laser beam emitted from the second light source 212 is reflected by the wavelength selecting film 218 and made incident on the second light source optical monitor 215 in order to control the optical output of the second light source 212. Accordingly, there is a problem that the quantity of light transmitted through the wavelength selecting film 218 and applied onto the optical disk 210 by the object lens 214 is reduced by the quantity of light reflected by the wavelength selecting film 218 toward the monitor 215. In other words, there is a problem that the data reproducing capability of the optical disk 210 (or data recording capability on the optical disk 210) using the second laser beam emitted from the second light source 212 is reduced by the reduction in the light quantity or intensity of the second laser beam incident on the optical disk 210.
Moreover, in order to avoid this problem, if it is attempted to design the reflectance of the wavelength selecting film 218 with respect to the laser beam emitted from the second light source 212 to a smallest possible value, for example, a small value of about 5%, there arises another problem as follows. That is, according to the current mass production technology, the reflectance of the wavelength selecting film 218 has a variation of about ±4%. Therefore, if it is attempted to set the reflectance of the wavelength selecting film 218 with respect to the second laser beam emitted from the second light source 212 at, for example, 5%, then the reflectance is varied within a range of 1% to 9%. For this reason, the dynamic range (a ratio of the minimum value to the maximum value) of the quantity of light of the second laser beam, which is emitted from the second light source 212, reflected by the wavelength selecting film 218 and made incident on the second light source optical monitor 215, has about ninefold variations, and this leads to a failure in accurately detecting the optical output of the second light source 212.
There is a further problem that, when the reflectance is varied to the smaller values, then the quantity of the laser light incident on the second light source optical monitor 215 is reduced, and the S/N ratio (signal/noise ratio) of the optical signal in the second light source optical monitor 215 is reduced, resulting in difficulties in detecting the quantity of the optical output of the second light source 212.
Moreover, if the wavelength λ2 of the second laser beam of the second light source 212 fluctuates due to a temperature change or the like, then the reflectance of the wavelength λ2 of the second laser beam on the wavelength selecting film 218 fluctuates, and the quantity of light incident on the second light source optical monitor 215 is to fluctuate. In general, if the reflectance is set low as described above, a fluctuation in reflectance due to a wavelength change is increased, and this leads to a problem that the quantity of light of the second light source 212 cannot be accurately detected by the second light source optical monitor 215.